I had problems with a Tatone EM-4 09-19 Calumet muffler. The Enya .19-VI TV ran very hot with it, it was very restrictive. Well, I turned lemons into lemonade. Trimming the inner baffles did not help, so I turned it into a tongue muffler. Now I have a light weight muffler for the Testors McCoy .19 Red Head.

It works, but I don't have a DB meter, so I can't quantify effectiveness. IOW, you are on your own, YMMV.

Designed an airplane for the K&B 4011. I did not leave enough space between the end of the muffler and the wing. I measured the area of the muffler outlet. I drilled enough 3/32 holes in the muffler to give me about 10% more area than the original outlet. I cut off the original outlet and plugged the hole. It worked fine.

In this case the muffler has a very small expansion chamber, almost next to none. One doesn't get the adiabatic expansion with each exhaust puff of the engine that would happen with a standard muffler. I ended up doubling the 1/4 inch (6 mm) diameter exit. Since I will be using it on a 33 in. (838 mm) wingspan Midwest P-40 Warhawk profile, it will spend its time 4-2 cycling at a slightly lower RPM. I'll know more when I fly, but it should be acceptable.